for some time now, I have carefully followed the wonderful discussions in the various EuroBricks forums on the usage of power systems for electrification of LEGO models, particular the ones in the TrainTech section. Every now and then, there appear to be serious “How-to” questions with regard to 9 V/PF train “conversions”. Here are some comments about this issue, focusing on the LEGO Lithium polymer rechargeable battery (#8878) for application in electrified LEGO train models (Note: Sorry for using BrickShelf deeplinks, just uploaded the stuff ...)
Generally speaking, TLC appears to push the various PowerFunction elements as a more or less global approach for electrification of LEGO models, at least in the current “9 V domains” (Technic, Mindstorms, RC-Train), right? Recently a PF-type train motor appeared and in fact, the prefix “PF” appears to be well justified – as Philipe Hourbain has very recently demonstrated. After reading this article by Brian Williams in RailBricks, Issue 6, page 17, I was finally convinced that $50 may be worth spent for some LiPo “research”: Is the LEGO LiPo really that, a full blown real world “LiPo” or does it need to be purchased in pair with the LEGO AC charger (#8887) and be carefully treated like a raw 10 V egg? In other words: How does it behave electrically? What can we really do with it?
In short: The LiPo meets expectation in every regard! Here are the results from some measurements. To really benefit from the superb LiPo performance however, DC track power-pickup is necessary. This has been amply discussed in EuroBricks; I prefer modified 9 V train motors, where the original “9 V motor only” function is fully restored when a classical LEGO 9 V cable (with two 2x2 electrical plates) is connected to the terminal of the motor. Two more custom cables (DC pickup to LiPo with bridge rectifier, and PF receiver to modified 9 V train motor) are required as well.
There will be a full article about the LiPo performance in an upcoming issue of RailBricks – with all the details on cables, “test-bed” set-up, etc. (Basically, the approach is completely analog to my RCX stuff shown here (page 53, figure 9. Here, the RCX charging cable does not need to have a bridge rectifier though, TLC already built that into the RCX, very smart!).
Here we go:
1) The 7.4 V/1.1 A LEGO PF Lithium polymer rechargeable battery can be charged with a broad range of DC input voltages ranging from 9 V to 18 V, see figure below. 10 V or above is recommended, since the then available full charging current is minimizing charging time. 12 V seem to represent the best price-performance ratio, as judged from the availability of cheap “wall wart” type power supplies. In other words: You don’t need to buy the way too expensive 10 V LEGO charger! I believe, 10 V is simply a good marketing joke (who on earth else is using 10 V? Oh yes, sorry I forgot: Safety, safety, safety. Well, at least you can’t swallow cheap wall warts that easily, and the power cord can probably be viewed as a safety pull-back mechanism. Ha, another one: Google for “10v dc power”. Hit #1: LEGO S&H … they are smart at TLC, aren’t they).
2) The maximum LiPo charging current was calculated in this MBFR forum entry, replies #3 and #4 (you would need to polish up your German though ..., but I bet there are many relevant forum entries at EuroBricks as well) before to be around 750 mA average, which was based on information from the LEGO service department and the fine print on the LiPo (1 ½ hours charging time, 1.1 Ah LiPo capacity). The present measurements however suggest a maximum charging current of 550 mA, even with two stalled XL motors present either directly on the output of the LiPo or a connected PF receiver output, respectively, set to full forward, see figure below. So when your cheap wall wart can lift that, you should be fine.
Figure 1: LiPo DC input voltage vs. LiPo EDIT
At 20 V DC-in the LiPo "turns off", and can't be re-animated anymore. Well, I did some measuremenst on the PCB, see Philo's photos here, everything seemd ok, but it still played dead. I waited 4 days, and then: Bingo, back alive, however with fully discharged battery. Charged it at 12 V, did it again, it "died" again at 20 V, same procedure - the LiPo is doing well again. A-m-a-z-i-n-g!
3) This suggests that for electrification of large train layouts with constant DC new or used computer power supplies are perfectly well suited; they deliver easily well above 10 A on the 12 V line. 12 V car lead batteries would also work well for many, many hours at high total track currents and they can be recharged with relatively cheap power supplies, e.g. overnight. There are many more options.
4) The LiPo works well as uninterruptible power supply (UPS); the output characteristics remain pretty much constant, regardless of charging voltage. The transition from DC to battery power is smooth and without any problems. There are some reports on issues with the interruption of the charging voltage (i.e., LiPo turns off). This has been observed in the present measurements only, when the charging plug was forcefully removed from the LiPo, but never when the charging voltage was simply turned off (e.g., by driving a DC pickup train from powered to non-powered track). The figure below shows the performance of a test-bed train on a layout with two reversing loops (short DC blackout, polarity change), some RC tracks (power outage), and mostly powered 9 V track present.
Photo of priciple of operation (the charging line has an SMD-type bridge rectifier):
Photo of train test-bed:
Figure 2: Round time vs. total run time of the test-bed shown above (Hybrid 9V/RC train, power pickup, PF LiPo, PF receiver, 10 cars, speed level "3", both motors on one PF receiver output. Red curve: Fully charged LiPo, no track power. Run time slightly above 4 hours. Blue curve: Starting with entirely flat LiPo from red experiment, with 12 V track power. LiPo seems to net-charge over time, since round time eventually goes down. After 7 hours, 12 V DC was removed: Indeed the LiPo was almost fully recharged, it ran for another 3 hours
The LiPo is a PF element which readily combines the best of both worlds, PF and 9 V train. DC power pickup is required in some way though.
Due to the relatively stable output of the LiPo, train driving characteristics are virtually constant when going over powered 9 V or RC plastic track.
More than one train can be operated on one stretch of powered 9 V track (I know, we all knew that already ...)
Charging of the LiPo is automatically enabled on powered stretches of 9 V track; no access to the charging jack is necessary. This means that your train design is less restricted; just completely hide the LiPo somewhere inside and stop the train on powered track.
What about using DC power from you 12 V train layout for feeding a LiPo equipped PF train as well? I have no 12 V tracks, so this is pure speculation, but should work well: The 12 V train system features a beautiful power pickup brick (#x552). If you run the 12 V via a bridge rectifier into a PF LiPo, you can use all your PF elements on the 12 V system as well … which then of course means that you may want to cross over to 4.5 V tracks without a glitch due to the USP function of the LiPo battery.
This is what I’d call “Building across multiple themes”. As far as I am concerned, TLC did it again.
There is more to come – the PF receiver is another very nifty piece of equipment …
Have fun + Play Well
Edited by Toastie, 03 September 2010 - 07:02 PM.